Catalytic Converters

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Abstract: Ceramic catalyst cores are typically wrapped in mounting mats made of ceramic fibers and packaged into steel housings. Specially developed materials and technologies allow the design and construction of extremely durable catalytic converters. A number of converter canning technologies were developed, including clamshell, tourniquet and stuffing. In each of the technologies, the converter shell geometry has to provide the required mounting density for the mat. The design of converter inlet and outlet headers or cones affects the gas flow distribution and pressure drop. In applications with space limitations, catalysts can be placed inside catalytic mufflers.

Introduction

Catalyst substrates coated with the active catalyst washcoat are packaged in steel housings to form catalytic converters. The following are major catalytic converter design considerations:

mechanical durability

heat losses

flow distribution

pressure drop.

Emission performance durability and mechanical durability are the two key aspects of the overall durability of an emission control system. The emission durability depends on the quality of the catalyst coating and on the operating conditions such as temperature or levels of catalyst poisons in the exhaust gas. The catalytic converter design has to provide the required mechanical system durability.

Catalytic converters must provide adequate protection for the substrate under harsh operating conditions in the vehicle’s exhaust system. Despite exposure to high temperatures and thermal shock, moisture and corrosive environments, as well as mechanical vibration (Table 1 [910]), they endure hundreds of thousands of kilometers. The final mechanical durability of the emission control system is a combination of the substrate durability, durability of packaging materials and packaging technology.

Table 1Operating Conditions of Catalytic Converters

Gasoline

Diesel

Temperature Range, °C

300-1100

100-650

Temperature Gradient, °C

100-300

100-200

Space Velocity, 1/hr

30,000-100,000*

60,000-150,000*

Vibration Acceleration, g

28

10-20

* - higher S.V. may be used in aftermarket applications

Since the diesel engine is more durable than its gasoline counterpart, the required life expectancy for diesel catalytic converters is also longer than that for gasoline converters. For example, since 2004 the US EPA durability requirement for emission control systems on heavy-duty diesel engines is 10 years/22,000 hours/435,000 miles (700,000 km), whichever occurs first (previously, the requirements were 8 years/290,000 miles or 467,000 km).

In situations where thermal losses from the converter are important, they have to be modeled during the converter design. Double walled designs with either air gaps or ceramic fiber insulation are commonly used on gasoline converters in the close-coupled location, which are optimized for cold start hydrocarbon performance. Since diesel cold start emissions are much less critical, such designs have not been used for diesel converters. However, due to the low temperature of diesel exhaust gases, diesel converters should be placed close to the exhaust manifold or exhaust pipe insulation should be applied to assure satisfactory catalyst performance. The low temperature performance, including cold start, is becoming increasingly important for diesel catalytic converters, especially in light-duty applications.

The geometry of converter headers, especially that of the inlet header, can influence the exhaust gas flow distribution in the catalyst. It is believed that flow maldistribution negatively affects catalyst performance and/or durability. That opinion, although not supported by convincing experimental data, became a widely accepted consensus. Even if the emission performance is not improved, a skillful design of the headers can certainly decrease the total catalytic converter pressure loss.

Catalyst canning technologies have evolved since the 1990s, driven by the demands of California LEV, ULEV and SULEV gasoline applications. Still more development will be needed to satisfy the demands of future exhaust systems, especially for diesel engines. The major factors responsible for the evolution in catalytic converter technology can be summarized as follows:

In gasoline applications, the converter was either moved to a close-coupled location, or a second close-coupled pre-converter was introduced in addition to the underfloor converter. The close-coupled converter, installed very close to the exhaust port of the engine, is exposed to high temperatures, high thermal shock conditions, and increased vibration, thus calling for more robust canning.

Ultra-thin wall ceramic substrates for gasoline engines have wall thickness on the order of 0.002-0.003" (0.050-0.075 mm). Example commercial configurations introduced in the late 1990s include 600/3 and 900/2 substrates. These parts are weaker than the older, thicker wall substrates (e.g., 400/6.5, of 0.0065" or 0.17 mm wall thickness). New materials and/or packaging methods were necessary to accommodate these parts.

Widespread use of catalytic converters on diesel vehicles, such as on Euro 3/4 cars, created specific challenges related to the low temperature operation.

Exhaust systems on heavy-duty engines, especially those incorporating diesel particulate filters (DPF), require very robust packaging. While monolithic DPF substrates are packaged using essentially the same methods as catalytic converters, they are much heavier and larger, thus creating new challenges.